PHPBuilder.com continues in their series of exceprt from the Sams Publishing book "PHP 5 in Practice" with this quick snippet showing how to create an interactive web application
by combining PHP and Ajax.

A full discussion of Ajax is not within the scope of this book and would be mostly a discussion of JavaScript, not PHP. However, the listings in this section demonstrate one quick example that shows how Ajax can call a PHP script, pass it data, and display the results back to the user.

Their simple example makes a "time service" that can be pulled from any remote script (in the domain, of course). The PHP for the backend is simple - just echoing out the current time, and the HTML/Javascript is simple too, making a basic XMLHttpRequest call to the backend script.